This invention relates to seismic source devices and more particularly to a shuttle and external sliding valve controlled seismic source device which allows only a portion of the air found in the firing chamber of the device from being discharged into the surrounding environment.
In marine seismic exploration, a source of acoustic energy is released into the water every few seconds to obtain appropriate acoustic waves that propagate into the earth's surface. These waves are reflected at interfaces of the subsurface formations and propagated back to instruments where transducers convert the acoustic waves to electronic signals which are recorded and later processed into a record section for interpretation of the subsurface formations. Marine seismic exploration is of two types, the first type being on water where the seismic source units are strung or towed from a water vehicle. The second type of marine seismic exploration is arctic marine exploration where the seismic source units are disposed below an ice layer to determine the formation of the rock surfaces below the ice layer.
During the past decade, the major marine seismic energy source for both types of exploration has been the air gun. An air gun as in the prior art releases high-pressure air (typically 2000 PSI up to 6000 PSI or even more) into the water to create the desired acoustic wave.
State of the art air guns normally comprise an annular housing that contains means for discharging compressed air through exhaust ports in the housing. Compressed air is stored within the housing in a firing chamber. The only moving component (except for the solenoid triggering device) in the state of the art air guns is a shuttle, which when raised, permits air to escape from the firing chamber through the exhaust ports in the main housing into the surrounding water. The size of the gun is determined by the firing chamber volume selected. By having a constant source of compressed air through an inlet passage in the housing, the upper chamber containing the shuttle is filled and forces the shuttle into a sealed position closing off all exhaust ports from the firing chamber. By using a solenoid valve to allow air flow underneath the shuttle flange thus forcing the shuttle upward and causing an unequal pressuring on the shuttle pistons opposing each other on the shuttle shaft, the shuttle is accelerated in the upward direction exposing the chamber exhaust ports and allowing compressed air to escape into the surrounding water. When the shuttle is in the down, or closed position, the air gun is charged and ready for firing. When fired, the state of the art air gun allows 80-90% of the air in the firing chamber to be exhausted into the water. Consequently, prior art air guns suffer two major disadvantages: first, the efficiency of the air gun for converting stored energy to useful acoustic energy in the seismic passband is well below 10%, and second, the undesirable secondary pressure pulses follow the first acoustic pulse and obscure or confuse the time of the reflected signals.
Several approaches have been taken by the industry to overcome this second disadvantage or undesirable secondary pressure pulses. However, it has been found that the solutions provided are either the cause for a less efficient system of converting stored to acoustic energy or result in greater expense in processing the data. Among the methods presently employed to reduce secondary pulse amplitudes are those that include throttling additional air into the bubble as it forms outside the chamber with a so-called "wave shaping kit", using an array of guns of different sizes, and thus different bubble pulse periods, to destructively reduce the secondary pulses and finally, "signature correction" techniques in data processing to reduce the secondary pulses recorded. "Signature" may be defined as the recorded wavelet or pressure level of the acoustic pressure discharged into the water over a fixed period of time.
Copending United States patent application, Ser. No. 929,883, entitled "Air Release Control in Seismic Energy Source Air Gun", filed July 31, 1978, and assigned to the assignee of this invention, teaches, as a preferred embodiment, an air gun incorporating a secondary shuttle which is activated at the proper moment to cause the closure, by a primary shuttle, of the exhaust ports to further discharge of air from within the air gun.
The present invention utilizes an external sliding valve structure activated by reduced firing chamber pressure resulting from the prior activation of the shuttle whose activation then causes covering of the exhaust ports by the sliding valve itself. The constant source of air pressure causes the shuttle to move downward, sealing off the firing chamber, causing the pressure therein to rise which in turn causes the sliding valve to move back down.